1. Field of the Invention
The present invention relates to a power-saving power control circuit for use to control the output of power supply to a lighting fixture and, more particularly, to an electronic power-saving power control circuit, which saves the consumption of power supply by dropping the voltage without affecting normal working of the lighting fixture at the load.
2. Description of the Related Art
Because the load of a lighting fixture, for example, a fluorescent lamp, is fixed, the design and installation of a lighting fixture are done subject to the nominal voltage of the power system, allowing the lighting fixture to function normally within a tolerance about ±5˜10% of the nominal voltage. The installation and brightness of a lighting fixture must be set subject to the voltage level below the nominal voltage but higher than the lowest level of the tolerance. The power consumption of an inductance ballast fluorescent lamp or the like has a direct concern with voltage level. The higher the voltage is the greater the consumption will be. Therefore, reducing the voltage to a lever closer to the lower limit of the tolerance greatly saves power consumption without affecting normal functioning of the load.
Various power-saving voltage stabilizers have been disclosed. These voltage stabilizers commonly use a voltage regulator to increase or reduce voltage, providing a stable voltage output to the load. However, these conventional designs are still not satisfactory in function because of the following drawbacks.    1. In order to fit high current, the voltage regulator is heavy, bulky and expensive, not economic for a small power system. Further, it is difficult to regulate the voltage to a suitable range. Excessive high drop of voltage causes the equipment at the load not able to function normally. Insufficient drop of voltage cannot achieve a significant saving of power consumption.    2. The voltage regulators of conventional power-saving voltage stabilizers cannot automatically and freely regulate voltage dropping. They drop the voltage step by step. The range between two steps may be too far, resulting in an unstable functioning of the load or insignificant saving of power consumption.    3. These conventional power-saving voltage stabilizers do not fit all equipment.
Further, power-saving designs using silicon controlled semiconductor to control output power to motor drives are well known. It is known that a motor drive consumes much energy than desired when the load is low. Using a silicon controlled semiconductor to control output power saves much energy. The saving of energy may be achieved by two methods, i.e., the variable voltage dropping and the fixed voltage dropping. According to the variable voltage dropping method, the dropping of output power and the range of energy saving vary with the change of the load, i.e., the output voltage varies with the voltage of power source and the load. According to the fixed voltage dropping method, the dropping of voltage is fixed subject to fixed input power voltage and fixed load, i.e., the output voltage varies with the input voltage. These two methods cannot be employed to lighting fixtures.